Scientists Find an Icy World Beyond Pluto, and the Solar System Suddenly Seems Bigger

By JOHN NOBLE WILFORD

Published: March 16, 2004

Far beyond Pluto, out where the Sun is only a pinpoint of pale light, a frozen world has been found on the dark fringes of the solar system. Astronomers say it is by far the most distant object known to orbit the Sun and the largest one to be detected since the discovery of Pluto in 1930.

With one discovery, it seems, the solar system has gotten much bigger, glimpses of its outer reaches bringing a sense of reality to what had been a remote frontier of hypothesis. And perhaps it has gotten stranger, too.

''There's absolutely nothing else like it known in the solar system,'' Dr. Michael Brown, an astronomer at the California Institute of Technology who led the discovery team, said of the newfound object.

But in a telephone news conference yesterday from Pasadena, Dr. Brown added, ''Our prediction is that there will be many, many more of these objects discovered in the next five years, and some of them will probably be more massive.''

The researchers, whose observations were supported by the National Aeronautics and Space Administration, said the object, referred to as a planetoid, is extremely frigid (minus 400 degrees Fahrenheit) and peculiarly red, probably more so than any other body in the solar system except Mars. They are not sure why, and also have few ideas of the object's composition. It could be a primordial mix of rock and ice.

Dr. Brown's group has proposed naming the object Sedna, after the Inuit goddess who created the sea creatures of the Arctic. For the time being, it is designated as 2003 VB12. The first sighting was made last November at the Palomar Observatory, operated by Caltech.

Sedna's remoteness has inspired scientists to conjecture over how much the discovery could be telling them about the far reaches of the solar system. The planetoid is more than three times as far from the Sun as the current distance of Pluto, normally considered the edge of the planetary system. But it travels a widely eccentric orbit, taking 10,500 years to revolve around the Sun.

Calculations by the researchers show that Sedna, now a relatively close 8 billion miles from Earth, wanders out as far as 84 billion miles, in a region presumably populated with icy bodies too small to be observed by telescopes.

Dr. Brown said Sedna ''is so far away from everything that it must be the first observed member of the long-hypothesized Oort Cloud, a sphere of comets out to halfway the distance to the nearest star.''

In 1950, a Dutch astronomer, Jan Oort, predicted the existence of a swarm of icy bodies stretched somewhere beyond the orbit of Pluto. The cloud is thought to surround the Sun and extend outward halfway to the next nearest star, Proxima Centauri.

The Oort Cloud is considered a repository of the comets that get pulled in toward the Sun. Dr. Brown said that though the danger is nil, ''if this object were to come into the inner solar system, it would be the most spectacular comet ever seen.''

Other astronomers, however, questioned whether Sedna should be considered a part of the Oort Cloud. In theory, the cloud's innermost boundary is charted to be well beyond the farthest point in Sedna's orbit. Some scientists suggested that Sedna could instead have been part of a closer region of cometary material, the Kuiper Belt, which stretches from Neptune to just beyond Pluto, and that it had somehow been dislodged and sent off on a more distant orbit.

Dr. Brown agreed that Sedna was much closer than expected for the Oort Cloud. But in a statement, he suggested that Sedna could reside at least part of the time in an inner sector of the cloud. The sector, he said, could have been separated from the greater cloud by the gravitational pull of a rogue star that came close to the Sun early in its existence.

Dr. Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics said the discovery was puzzling and exciting. ''You wonder where indeed did the object come from,'' Dr. Marsden said. ''Is it connected to an inner Oort Cloud? What does that mean and how did it get there?''

He said that it was likely that there were similar icy bodies beyond Pluto, and that some may be larger than Sedna.

Two years ago, Dr. Brown and his colleagues, Dr. Chad Trujillo of the Gemini Observatory in Hawaii and Dr. David Rabinowitz of Yale, found a smaller object, called Quaoar (KWAH-o-ar), which until now was the largest known object beyond Pluto. It is estimated to be about 40 percent as big as Pluto.

Even Pluto itself is thought by some astronomers to be less a planet and more like a Kuiper denizen. In fact, the larger Kuiper objects are sometimes called plutinos.

The astronomers got their first look at Sedna with a 48-inch telescope at Palomar, in southern California. Within days, other telescopes in Chile, Spain, Arizona and Hawaii made observations. When NASA's new Spitzer Space Telescope took a look with its infrared detectors, astronomers were able to make rough estimates of the planetoid's size.

To the other telescopes, the object was no more than a point of light. But infrared measurements of heat radiating from the object led researchers to estimate its diameter at no more than 1,100 miles. Pluto's is 1,400.

Dr. Trujillo said that the nature of Sedna's surface was a mystery and that its ruddy color was ''nothing like what we would have predicted or what we can currently explain.''

But Dr. Marsden said the redness was not necessarily a surprise. ''Comets, many of them, tend to be reddish,'' he said.

It was hard enough finding Sedna in the first place. In looking for small solar system travelers, astronomers need a sequence of pictures to reveal that an object has moved in relation to background stars. But an object like Sedna, with its 10,500-year orbit, seems hardly to move at all. Only by using archival pictures from two years ago were the astronomers able to detect and clock its millennial pace.

As the scientists calculated it, Sedna should reach its nearest point in 72 years and then begin heading back out to the far frontier of the solar system.

''The last time Sedna was this close to the Sun,'' Dr. Brown said, ''Earth was just coming out of the last ice age.''